The field of the invention is motor drive systems for large electric mining machines such as shovels and drag lines.
Large mining machines include three separate motor drive systems. In a shovel, for example, a first motor drive serves to hoist and lower a dipper which is fastened to the end of a handle, a second motor drive serves to crowd and retract the handle with respect to a boom, and a third motor drive serves to swing a revolving frame which supports the boom. In large shovels such as those disclosed in U.S. Pat. Nos. 3,690,483; 3,708,152; 3,901,341 and 4,053,139, Ward-Leonard drive systems have almost always been employed. Such drive systems include control circuits exemplified by that disclosed in U.S. Pat. No. 3,518,448 issued to D. E. Barber on June 30, 1970 and entitled "Control System for Excavating Equipment."
Ward-Leonard drive systems for mining machines include a large a.c. induction motor which drives a d.c. generator and the d.c. generator in turn provides electrical power to a d.c. motor. The d.c. motor drives the hoist, crowd retract or swing drive mechanisms of the mining machine and its torque and/or speed is typically controlled through a field winding associated with the d.c. generator as illustrated in U.S. Pat. Nos. 3,806,780 and 4,031,440. The size, weight and cost of a drive system having three rotating machines in the two hundred to one thousand horse power range is enormous and it has long been an objective in the field to employ a single a.c. or d.c. motor in place of the conventional Ward-Leonard system.
The use of an a.c. induction motor to drive the various mining machine motions is particularly attractive because they do not have commutating brushes which must be maintained and periodically replaced. To control the speed of an a.c. induction motor, however, the frequency and level of the applied voltage must be varied and this necessitates the use of inverter circuits which employ solid state devices such as silicon control rectifiers. Although such "static" a.c. motor drives have found numerous applications in other fields, their application to mining machines present a number of unique problems.
One such problem is the energy which is regenerated by the a.c. motor during certain portions of the digging cycle. For example, when the dipper is lowered the hoist motor regenerates energy through the inverter. If this energy is not dissipated, the voltage across the direct current supply for the inverter can rise to destructive magnitudes. In prior Ward-Leonard drive systems such regeneration presented no problem because the energy was simply regenerated onto the power lines.
Another difficulty with electric mining shovels of all types is the frequent power outages which occur at mining sites. Typically, mines are located in remote areas and power lines must be extended over long distances. Momentary power outages of less than one or two seconds are quite common and it is neither necessary nor desirable to shut down the mining machine when such momentary outages occur. On the other hand, excessive currents and resulting overheating of the drive motor can occur if it is allowed to operate for a prolonged period with reduced voltage. In prior Ward-Leonard drive systems, the kinetic energy stored in the rotating machines was often sufficient to maintain voltages at the required levels during momentary power outages and voltage sensitive disconnect switches were employed to sense a prolonged reduction in applied line voltage. Although capacitors can be employed in small static a.c. motor drives to store energy, the size and cost of such capacitors is prohibitive when applied to a large mining machine and other means must thus be employed to ride through momentary power outages.